1. Field of the Invention
The present invention relates generally to the field of rotary feedthrough seals. Particularly, the present invention relates to a rotary feedthrough seal assembly.
2. Description of the Prior Art
Rotary feedthroughs have been developed for use in closed systems that may be under vacuum or at atmospheric pressure or at elevated pressure. These seals are typically high pressure mechanical seals. The function of these rotary feedthroughs is to provide a virtually hermetic, rotating seal. There are basically two types of rotary feedthrough seals. These include mechanical seals such as, for example, where O-rings are positioned around a shaft or where magnetic fluid and its accompanying magnetic system are positioned around a shaft.
Magnetic fluid feedthroughs have been developed for use in vacuum systems where they perform the function of providing a virtually hermetic rotating seal. These products rely on the entrapment of magnetic fluid and its ability to withstand a pressure differential when magnetically trapped. Vacuum feedthroughs are designed to withstand low pressures on the order of 1 to 2 atmospheres and are impervious to gaseous medium.
A conventional design of a magnetic fluid feedthrough is illustrated in FIG. 1. A magnetic fluid rotary feedthrough 10 typically has a housing 12 and a rotary shaft 14 that extends out of housing 12. Shaft 14 is magnetically permeable and has two sets of stages or teeth 16. Housing 12 is typical of a conventional feedthrough using a standard vacuum compatible flange 18 for mounting the rotary feedthrough 10 to a vacuum chamber. Shaft 14 is usually supported by a pair of bearings 20 that are disposed on either side of the magnetic fluid seal 22. The magnetic fluid seal 22 consists of two stationary, annular, magnetically-permeable elements 24, which are referred to as pole-pieces. Pole-pieces 24 carry magnetic flux from a permanent magnet or magnets 26, disposed between the pole-pieces 24. The magnetic flux is concentrated at the shaft 14 by a series of stages or teeth 16 cut into shaft 14. The shape of these stages or teeth 16 can be rectangular or triangular. Magnetic fluid or ferrofluid 28 is trapped within the concentrated magnetic flux and acts as “liquid o-rings” at stages 16. Shaft 14 is rotated usually by an electric motor, which can be external or integrated into housing 12.
Magnetic fluids or ferrofluids are colloidal suspensions of magnetic particles in a continuous phase. Colloidal stability is achieved by the use of a suitable surfactant. The continuous phase is selected on the basis of the application. For a vacuum application, this is an oil with a very low vapor pressure. The volume fraction of the magnetic particles is low, typically around 5-14%. However, owing to the surrounding surfactant layer, the effective volume fraction is significantly larger than the particle volume fraction.
When magnetic fluid is trapped by magnetic forces, the liquid o-rings are able to resist an external pressure and therefore act as a seal. These seals are hermetic and, being non-contacting, do not produce contamination making them ideal for processes conducted within a vacuum chamber. The shaft is usually driven by a motor or some other means and rotary motion is transferred from the outside into the clean environment of the vacuum chamber.
Such feedthroughs have been shown to successfully operate from a few revolutions per hour up to several tens of thousands of revolutions per minute. Pressure capability extends from ultra high vacuum to many tens of atmospheres. Leak rates are almost immeasurable at values of 1×10−11 cc/sec or lower. Operating temperature vary from as low as −55° C. to +200° C. with suitable protective measures.
Magnetic fluid or ferrofluid rotary feedthroughs are commonly used in vacuum systems where a hermetic, non-contaminating seal is required; systems typified by semiconductor, precision optics and solar cell manufacture. In all cases, these rotary feedthroughs are high precision components utilizing precision bearings and often including integrated motors and encoders. The systems they are typically used on are high value capital equipment and the expected uptime of the entire system can be as high as 99%. Many of these systems, particularly in the semiconductor environment, are operated under clean room conditions where entry by people is restricted.
The maintenance of a good vacuum in the equipment is essential and the rotary vacuum feedthrough is regarded as an item of critical importance. While the reliability of these feedthroughs is excellent, there is still a need for preventative maintenance. There are circumstances such as over temperature or contamination where the integrity of the feedthrough can be compromised.
In the case of solar cell manufacture where rotary vacuum feedthroughs are an integral part of the transportation mechanism and where there can be many hundred of such feedthroughs in operation on a single machine, it is often very difficult to identify one feedthrough in many where there is a problem with feedthrough integrity. This is particularly difficult in the case of an intermittent or sporadic fault. Often, the only evidence that a feedthrough is leaking is when atmospheric contamination of the product is observed rendering it unusable. Additionally, the offending rotary feedthrough needs locating and replacing, which is not a trivial exercise in a chamber that could be as much as 100 meters long with over 200 rotary feedthroughs. Once the offending feedthrough is located and replaced the chamber needs to be re-evacuated, a process that can take many days resulting in a loss of production.
Therefore, what is needed is a rotary feedthrough that is more easily maintained. What is also needed is a rotary feedthrough that is more easily identified when the rotary feedthrough is about to fail.